The present invention relates generally to an improved corrosion resistant systems for use in with high density, high performance drilling and completion fluids and in particularly those high performance drilling and completion operations that utilize formate brine systems. Formate brine systems are frequently employed in conjunction with drilling operations to aid and facilitate the drilling operation. Of particular interest are those downhole or slimhole drilling operations which are frequently utilized in offshore drilling. The use of formate brine systems in such drilling operations, while advantageous for expediting the operation, have also been found to cause some degree of general corrosion as well as stress corrosion cracking (SCC) on steel.
Alkali bromate technology is finding use in areas where environmental concerns are paramount particularly in arctic regions such as offshore northern Norway. The alkali-metal salts of formic acid, particularly potassium formate is readily available, non-hazardous, and readily biodegradable (70% degraded in 28 days). As indicated above, corrosion studies have shown the occurrence of some general corrosion and also some slight corrosion cracking (SCC) effects on steel. It has been found that a significant reduction in the corrosion normally occurring in the system may be achieved by means of blending the alkali salts of formic acid such as potassium formate with inhibitors that are environmentally friendly and function in combination with the alkali bromates employed in drilling operations.
In order to perform the function of inhibiting corrosion in the severe service conditions of drilling, particularly downhole drilling, effective corrosion inhibitors must have the capability of forming durable films on the surfaces of the steel components. Because of the harsh conditions which must be met, the combined film forming and corrosion inhibiting functions must be achieved without creating conditions which could be expected to cause a deterioration in the environment.
It has been determined that certain corrosion inhibitors advantageously added to alkali formate brines including potassium formate brines serve to control corrosion in the high pH basic carbon dioxide environment where the formate brines are being utilized in the drilling operation. The corrosion inhibitors of the present invention and which that are most useful combine the function of corrosion inhibition with formation of durable and tough films on the various metallic surfaces present in well drilling and operation.
The alkali salts of formic acid in drilling fluids have the unique feature of being environmentally friendly this being a rigid requirement in offshore drilling in the Artic regions. The high density formate fluids also facilitate faster drilling, as the plastic viscosity of the mud becomes lower due to the high solubility and density of the bromate salts. The effect of the equivalent circulating density and the lower downhole pressures result in greater drilling efficiency. This is very important in downhole drilling where pipe diameters are necessarily reduced and downhole pressure losses must be controlled.
Because of the high pH of the formate systems, steps must be taken to maintain the pH within practical limits. This requires constant buffering with carbonate salts in order to maintain solubility. Even with the use of such buffered solutions a certain amount of corrosion and pitting arises on the drilling equipment. In accordance with procedures undertaken pursuant to the present invention, tests have determined that selected inhibitors employed in connection with the invention are very effective in these systems.
In accordance with the present invention, it has been determined that when exposed to conditions encountered in downhole drilling, potassium formate brines blended with selected inhibitors of the present invention significantly reduce the rate of corrosion on both carbon and chromium steel. In certain applications, small amounts of these inhibitors in buffered potassium formate solutions virtually prevent the occurrence of corrosion. In certain applications, blends containing more than about 0.3% inhibitor reduce the corrosion on chrome steel to less than 30% of that of the control, while with 0.5%, the corrosion was reduced to 0.2% of the control. Similar reductions were experienced with carbon steel.
Non-buffered potassium formate/inhibitor solutions of the present invention yielded similar corrosion rate reductions. Test results clearly demonstrated that the combination of buffering and the addition of inhibitor to the alkali formate solutions provided the most effective corrosion control system for downhole drilling applications. More importantly, results showed that reductions of corrosion rates in both buffered and non-buffered alkali formate solutions are achieved by adding selected corrosion inhibition molecules to the formate solutions.
Therefore, it is a primary object of the present invention to provide an improved system for use in oil drilling applications, particularly downhole drilling applications, wherein a drilling fluid is formulated utilizing an alkali formate brine together with selected environmentally friendly corrosion inhibitors, with the resultant fluid being highly compatible with drilling operations.
It is yet a further object of the present invention to provide an improved drilling fluid adapted for use in offshore drilling operations, particularly downhole drilling operations wherein the drilling fluid comprises buffered alkali formate solutions and a corrosion inhibitor comprising an amido amine salt formed as a reaction product of fatty acid derivatives.
It is yet a further object of the present invention to provide an improved corrosion resistant drilling fluid system which employs fatty acid amido amine salts in combination with a buffered alkali formate solution.